This invention relates to a process for drying water-wet polycarbonate membranes.
The use of polymeric membranes for gas separation is well known. A wide variety of polymers have been used for gas separation membranes, including cellulose esters, polyamides, polyimides, and polyolefins. An application of particular interest is membrane separation of oxygen and nitrogen from air. For example, enriched nitrogen streams obtained from air may be used for inert padding of flammable fluids or for food storage; enriched oxygen streams obtained from air may be used for enhancing combustion or for increasing the efficiency of fermentation processes.
The membranes used for gas separation are generally dry so that the most effective membrane separation performance can be achieved. However, many membranes are formed by the wet process, in which a solution of polymer, solvent(s) and optional non-solvent(s) is cast or extruded, the solvent(s) and non-solvent(s) optionally allowed to partially evaporate, followed by immersion in a coagulating liquid bath, often water. Thus, the membranes formed by the wet process are liquid-wet and preferably are dried prior to use for gas separation. The art teaches that care must be taken during the drying process to maintain the physical structures of the membranes because structural changes such as pore collapse or crazing result in adverse membrane performance. The art discloses several techniques for drying water-wet cellulose ester membranes so that the physical structures of the membranes are preserved. One such method is freeze drying. Another method involves sequentially contacting the cellulose ester membranes with polar and non-polar solvents. The purpose of the sequential solvent method is to sufficiently reduce the polymer-water interaction by replacing water with a non-polar solvent, thus lowering the surface tension, so that the membranes may be dried without an adverse impact on the structures of the membranes. The problem is that such techniques are expensive, time consuming, and generate large volumes of solvent for disposal. Furthermore, such techniques often introduce sources of variation in membrane performance.
Polycarbonate membranes in particular have been found to have good separation properties for oxygen and nitrogen. Polycarbonate membranes formed by the wet process generally are porous or asymmetric, depending on the extrusion or casting conditions. Porous membranes may be used as supports for composite gas separation membranes. Composite membranes possess a thin, dense discriminating layer. Assymmetric membranes possess a thin, dense discriminating layer supported on a porous substructure of the same material. The discriminating layer provides the membrane with gas separation capability. The membrane discriminating layer is preferably as thin as possible while still maintaining the ability to separate gases in order that the highest possible gas flux through the membrane may be achieved. POWADIR membranes may also be fabricated by the wet process. POWADIR membranes possess one or more discriminating regions capable of separating gases and one or more porous regions. An asymmetric membrane is a POWADIR membrane, but a POWADIR membrane is not necessarily asymmetric.
Polycarbonate membranes formed by the wet process may be directly dried in air. However, such polycarbonate membranes generally contain small amounts of residual solvent and non-solvent even after leaching and annealing which adversely affect the performance of the dried membranes. The presence of even small amounts of residual solvent and non-solvent in the dried membranes can result in reduced gas flux, reduced separation factor (selectivity), and increased compaction rate. An inexpensive, timely, and reproducible method of drying polycarbonate membranes which enhances separation properties through the removal of residual solvent and non-solvent prior to drying is needed. Furthermore, polycarbonate membranes formed by the wet process may possess a lower than optimal gas selectivity because of microscopic deficiencies in the membrane's morphological structure. For example, the discriminating layer may contain microscopic defects interrupting the continuity of the discriminating layer, resulting in a less than optimal gas selectivity, or the discriminating layer may not be "dense" enough, that is, the pores in the discriminating layer may not be sufficiently small so that the discriminating layer is capable of efficiently separating gases. Therefore, a process is also needed which results in increased gas selectivity through a modification of the membrane's morphology by "tightening" the discriminating layer without producing a significant decrease in the gas flux through the membrane. A single process which results in improved membrane separation performance through both removal of residual solvent and non-solvent and modification of the membrane's morphological structure would be particularly advantageous.